Ductile cast iron and method of making the same



jan 2 w34 if. A. FAHRENWALD 1,941,671

DUCTILE CAST IRON AND METHOD OF MAKING THE SAME .Filed Jan. 6, 1930F.3d. i

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UNITED STATES 'PATENT or-Flcsf .i 1,941,671 l f DUCTILE-CAST IRON ANDMETHOD 0 MAKING THE SAME Frank A. li'alirenwald,A Chicago, lll., signorto The American Brake Shoe and Foundry Company, New York, N. Y., acorporation o! Dela- Application January 6, 1930. Serial No. 418,7@ 4claims. (ci. 14s-21.1).

This invention relates tocast `iron articles, regardless of weight orsize but more especially to those articles which are of large size, andit has for' its objects the provision of increased strength Withoutincrease of weight or substantial increase in cost; the provision ofcast iron articles retaining the same chemical composition and the samefoundry practice as at present but with4 chined; while further objectsand advantages ofthe invention /will become apparent as the descriptionproceeds.

An example of the utility of my invention is presented in connectionwith manhole covers such as are employed for street ko1' pavement use.The large size of such devices` necessitated by the constantlyincreasing bulk of the articles required to be handled through theopenings, combines with the constantly increasing weight and speed oftraffic to demand a degree of strength in the cover which can beobtained in cast iron only with difhculty, and in any event at theexpenseof such weight as renders the device ex- Ordinary malleable castiron is scarcely suitable for the purpose owing to its deficiency intensile strength even if it were `available on a cost basis; and thesoftness of malleable iron would cause the original roughness thereof tobecome obliterated with the production of an excessively smooth plate onwhich animals and vehicles would slip.

In the drawing accompanying and forming a part of this application Ihave shown for purposes of illustration ythe application of my inventionto the production of a manhole cover,

though without intent to limit myself thereto.-

Fig. 1 is a perspective View of the completed article; Fig. 2 is apartial sectional view; Fig. '-3 is a sectional view through themoldready for the reception of the molten metaly-Fig. 4 is acharacteristic photomicrograph VAat the pointy a of the resultingcasting Fig. 2; Fig. 5 is a characteristic photomicrograph at the point'b of vsuch casting Fig. 2; Fig. 6 is a time-temperature diagram of thefirst heat treatment; Fig. 7- is a characteristic photomicrograph at thepoint a of the same casting after the rst heattreatment; Fig. 8 is atime-temperature diagram of the second heat treatment; and Fig'. 9 is avcharacteristic photomicrograph at the point a' after the` second heattreatment. The magnication illustrated in the drawing is approximately400 diameters, but it will be understood that the drawing is somewhatidealized owing to the dimculty of copying with photographic accuracy.v

Inv the production of vmy improved cast iron articles I employ what Iterm an unstable ironmixture, namely one which will cast gray in a sandmold but white under chilling conditions.

Examples of such mixtures. are as follows:

3.25153. 215mm zwaan .omaggi izsmhvg,l motero# {Balance {Balan {Balan.somo .como .lmao .zumo .zown .amo 1.oowo Loowo i.oowo,

I prefer mixture B, although the others can 7 be used. All of them willcast gray throughout in a sand mold and white to a considerable depthwhen chilled. I cast this mixture according to usual foundry practicebut in a mold made partly of sand or other non-chilling material andpartly of metal, (although in some cases it is possible to make the moldall of metal) so that the resulting casting shows the characteristiccondition of chilled white-iron in those parts whichv are adjacent tothe metal portion of the mold, and gray-iron in those parts (if any)which harden in contact with sand. For example, if it be desired to makea fiat, circular manhole cover 1 as shown in Fig. 1, havingstuds 2 ontop, I form from. sand 3 that part of the mold which defines the studs2, as for example, the portion 4 of the iiaskwhich islmown as the "drag,and employing in lieu of the usual'cope a.v

massive block 5 of suitable metal such as ordinary. cast iron. And thepouring can be effected in any suitable manner, the method here employedconsisting of forming the metal block 5 with a n of heat andconsequently ofv uidity, and of com' ing into contact with the metal 5and hence undergoing chilling only when the mold has become completelyfilled. The mold may be positioned in any mannerv desirable for thecasting* operation.

It is well known among foundry lnen that by slightly changing therelative proportions of the carbon and silicon, any desired depth ofchill can be obtained, and my preference is, with a castingof thecharacter described, to chill it approximately one-half way through, asshown at 7 in Fig. 2, the remainder of the thickness consisting ofgray-iron, and indicated at 8 in Fig. 2, except for a narrow transitionband or layer at 9 which shows an intermediate or mottled condition.

Metallurgically speaking the conditions exhibited Vby the portions 7 and8 are as follows: any-microscopic section of the portion 7 shouldexhibit no primary graphite but all the carbon'should occur in the formof rounded masses 10 (Fig. 4) consisting of iron carbide (known ascementite) submerged in and embraced by a continuous metal phase 11having substantially the characteristics of an excessively high carbonsteel rendered excessively brittle by the sudden quenching from amelting temperature caused by contact with the metal chiller 5. Anymicroscopic section of the portion 8 should exhibit little or no primarycementite but instead a very large amount of primary graphite" in theform of thin flakes or scales 12 y(Fig. 5) so closely packed as almostto interrupt the metal phase, and so large ordinarily as almost to bevisible to the naked eye. These graphite scales or flakes or plates aresurrounded and embraced by a continuous metal phase 13 consistingsubstantially of annealed steel and generally exhibiting a distinctpearlitic structure as attempted to be shown in Fig. 5, which consistsof alternating minute layers of secondary cementite and of pure ironknown as ferrite. By the term primary I refer to a formation produced inthe first cooling of the molten metal. By the term secondary I refer toa formation produced from a change in a primary constituent. Ordinarilythe portion 8 will be entirely free from cementite excepting that whichis contained in these minute layers; and the reason for the differencebetween the two portions of the casting arises solely from thedifference in the cooling `rates which in the portions adjacent to thesand affords sufficient time for the cementite to become broken downwith the` liberation of graphite and ferrite, and the metallic phase toarrange itself in layers as shown in Fig. 5, while in the portionsadjacent to the chiller the cementitehas no time to break down nor doesthe metal phase have an opportunity to become segregated into layers.

,If such a composite casting as I have just described were allowed tocool to room temperature' it would probably break into pieces (andperhaps explosively after it fell somewhere below about 600 Fahrenheit);but instead of allowing it to cool in this manner I preferably transferit lmmediately to a furnace where it is maintained at some temperaturebetween 1650 Fahrenheit and 1850 Fahrenheit for a sufllcient length oftime to decompose the primary cementite portions. This decomposition isa function of the temperature and the time. the curves of Fig. 6 showingthe approximate relation of toughness to time obtainable with `thedifferent temperatures indicated, the composition being the same in eachinstance. Owing to the essentially unstable character of the chillediron, which has retained its carbon in the combined form only because ofthe rapidity of its solidiflcation, this decomposition occurs veryreadily and its rapidity is remarkably influenced by the temperature.Thus in certain tests I have made at a temperature of 1800 Fahrenheitthe decomposition of the cementite was'.

foundl to be complete in less than one hour; at 1700 Fahrenheit it waspractically completed after eight hours; at a temperature of 1650Fahrenheit it was well begun after eight hours; and at 1800 Fahrenheitwas scarcely noticeable in eight hours. I'he result of this treatment isindicated in Flg.7, each of the primary cementite grains being replacedmore or less completely by a nodule 14 of pure graphite which occupiesthe same position and exhibits very much the same outline. Thisdecomposition appears to start inside each cementite grain andprogresses toward its exterior and so far as I have been able todetermine the composition of the matrix or metallic phase 11* betweenand around those grains remains unchanged until after the massivecementite has been decomposed; except that due to the fact that thetemperature is above the critical (1325 F.), this continuous phaseduring the heat treatment consists of austenite (carbon dissolved iniron) with a carbon content characteristic of the temperature employed.If the heat be conf tinued over long, the strength was found tofall olfvery remarkably as shown in; Fig. 6. The7 maximum strength obtainable ata temperature of 1800 Fahrenheit is developed at considerably less thanone hour, and owing to the rapidity of the reaction and thecorresponding rapidity of the subsequent decline in strength I have beenunable to measure the same accurately. At 1700 Fahrenheit, the maximumstrength appears to be developed in about four hours and to fall oiTappreciably after about six or seven hours. not determined the maximumstrength obtainable at the lower temperatures or the time required,since it is smaller in amount than is obtainable at 1700 and the longertreatment would be uneconomic. The curves shown in Fig. 6 areillustrative rather than exact since different compositions show diversevalues though all behave in this general manner.

The theory whereby I account for this loss of strength is largely thatthe comparatively great 1 masses of graphite produced by thedecomposition of these large cementite grains exercise a proaregenerally less susceptible to decomposition than the more massivecementite closer to the chilled face.

If this casting be now cooled gradually to room temperature it will befound notably superior in strength to ordinary gray cast iron, the lowerlayer being tough and tenacious and better fitted to bear the strains ofuse, the top or wearing portion retaining the characteristic wearingqualities of the gray-iron. The cooling rate which I designate by theterm gradually is any rate which shall avoid quench-hardness of thematrixr metal 15, since the austenite at 1700 or thereabout,`

and in the presence of so much available graphite, will become sosaturated with carbon that sudden cooling will render it brittle. Toprevent this I have or by piling the articles in thermally insulatedpits, or such procedures as are employed by the makers of malleable ironafter the termination of the heating stage. 'I'hese procedures willimprove the product of my invention whatever may be the effect withmalleable iron; for malleable castings are originally made of hardwhite-iron cast in molds, that is to say, of stable" whiteiron, and thetemperatures and times exhibited in Fig. 6 are scarcely sull'lcient toproduce any visible decomposition, while the amount of heating necessaryto decompose the stable vcarbide produces also a decomposition of thematrix metal so that the result of this long continued heating, coupledwith the long continued cooling produces a matrix of almost pureferrite. But the unstable iron after subjection to the short heattreatment I have described still exhibits a matrix metal oftrue/"austenitic composition the same as in the portion 8, but differingfrom the portion 8 in that whereas in the latter the graphite scales arevery tiny and thin and so closely adjacent as to leave very little metalbetween, in the former they-are fewer, larger, more rounded and furtherspaced so as to present a much less interrupted phase.

This austenite has the following properties; it can normally exist onlyat temperatures above about 1325" Fahrenheit known as the criticaltemperature), since iron does not normally hold carbon in solution butabove that temperature it exhibits a dissolving power for carbon whichvaries directly with the temperature; and aus-` tenite when mixed withgraphite as in these lnmass exhibits microscopically the finelayeredarrangement shown at 13 in Fig. 5, the same comprisinginnumerable plates or scales of secondary cementite interspersed withand embraced by a continuous phase of ferrite. This is thecharacteristic condition of eutectoid pearlite steel. Il' reheated abovethe critical temperature the ferrite and cementite portions mergetogether again forming austenite,if suddenly quenched from thisaustenite condition so as to prevent this segregation into layers acondition of great hardness is produced known as troostite, martensite,etc. and illustrated at l1 in Fig. 4. If austenite at a highertemperature than the critical, and saturated with carbon at such highertemperature, be suddenly quenched before it has an opportunity to ejectany of this carbon, a still higher degree of hardness and brittleness isproduced. Hence. if a casting after the heat treatment I have describedbe cooled with undue quickness the austenite, which hasv becomesaturated with carbon at whatever temperature was employed for such heattreatment, say 1700 Fahrenheit, will be converted into a steel of verygreat hardlness and so brittle as to be deficient in shock-I resistingability, but if cooled more gradually much of this excess carbon willbeejected and ypresumably absorbed by `the graphite nodules until thecritical temperature is reached, after which the pearlitic structurewill be assumed by the remaining ingredients. This condition is veryreadily produced by removing these articles from the furnace andstacking them in vertical cylindrical pits formed in. a bank ofsilcocel, kieselguhr, magnesium or even sand, where they are left forone or two days. It should be noted that these considerations relativeto the composition, heat treatment and microphysical structure ofthecontinuous phase apply theoretically to the gray-iron portion 8 aswell as to the modified white-iron portion 7, but practically theinterruption of the continuous phase by the graphite plates or grains isso nearly complete that the modification of strength possible to beachieved in the gray-iron portion is very small as compared with vthatobtainable in the portion 7 wherein the continuous phase is much lessinterrupted. The nodular arrangement of the graphite characteristic ofthe first heat treatment I have described renders it possible however byan elaboration of this second heat treatment to enhance even further thestrength of the resulting article. This, in its preformed` form,comprises the successive temperature-time relationships graphicallyrepresented in Fig. 8. The article having been removed from the furnacewhere it was rfirst heat-treated, for example at 110 a temperature of1700" Fahrenheit, is allowed to cool to the point C which'is preferablyimmediately above the critical temperature, for example, l350Fahrenheit, (or even nearer if the apparatus employed be susceptible ofsuch close control). It is kept at this temperature fory such length oftime as to enable the carbon content to become normal for thattemperature, whereupon the articles are quenched quickly through thecritical temperature as shown at D-E. The m best mode of effecting thisis by immersing the same briefly in a bath o! molten metal, for examplelead, which may be at any temperature above its melting point since itis permissible to cool the article even as low as 600 Fahrenheit 125 asindicated by the point P' though I do not recommend the practice. Coldair jets or oilv or water spray can also be used for this quenching,though the metal bath is preferable. This produces an Vextremely hardand somewhat brittle condition, -to alleviate which the article isimmediately introduced into a succeeding cham-V ber where it ismaintained for a suitable length of time at a temperature indicated bythe horizontal line E-H, which is always less than the criticaltemperature, but preferably not very much less, although it may fall aslow as 900 to 1000n Fahrenheit if the time of treatment be sui'licientlyprolonged. The result of this treatment is to allow the cementite, whichexcepting 14a for the quickness of the quenching would produce thelayer-like arrangement characteristic of pearlite, to become segregatedfrom the matrix metal, which it does, not in the form of sharp edgedplates characteristic of pearlite, but of tiny rounded masses orglobules 15 (Fig, 9) embraced by `a continuous phase 16 of ferrite; anddue to the greater continuity of this material and its essentialtoughness, the resultant article, alta' being. cooled to atmosphericl5.)

temperature, exhibits a degree of strength and toughness heretoforecharacteristic only of teinpered steel, although a steel impregnatedwith rounded masses 17 of secondary graphite.

The times and 'temperatures employed are susceptible of considerablevariation. 4vIthave found a suitable and very satisfactory condition forthe treatment indicated at C--D to befa furnace chamber maintained at1350ov Fahrenheit for one hour; the quenching D-E for four minutes in alead bath at 800" Fahrenheit, and the treatment E-H (or F-H) to be fortwo to four hours at a temperature of 1300 Fahrenheit, (or for ten hoursat 900 Fahrenheit). However, it is entirely permissible to omit thefirst chamber and cool the article gradually along the curve M,quenching as soon as the critical temperature is approached, or to coolalong some other curve as shown at N to a temperature indicatedat O,which is higher than the critical, and quench from that point, the onlydilculty being that the metal phase then contains alsomewhat higherpercentage of carbon necessary to be agglomerated. This first chamberwhen used can also be kept at any temperature up to about 1450 or even1500" Fahrenheit although the higher temperatures are less desirablebecause of the increased amount of carbon to be subsequently disposedof. An important advantage ofusing the first chamber, however, is thatit overcomes any disadvantage due to the accidental cooling of thearticle under the critical point as shown at P. In fact the article canbe cooled clear to room-temperature, and subsequently treated by thislast refinement of my general process, since any elevation of thetemperature above the critical will produce the austenitic condition,and it is imperative that the line E-H must never cross the 13.25Fahrenheit line else a new quenching will become necessary.

Theoretically this heat treatment last described has also strengthenedthe continuous phase or matrix metal of the portion 8 in the same way asthat of the portion 7, but practically its advantageous effect iscomparatively small, due to the extent to which that phase isinterrupted by the graphite flakes, so that its practical eiect islargely confined to the matrix metal of the portion 7.

I have described my improvemnt with especial reference to manholecovers, since these are simple in shape, exacting in their requirements,and manufactured in very large volume; but my invention is not by anymeans so limited. It may be applied to the manufacture of grate bars forlocomotives and other furnaces; stoking and other fuel burning devices;catch basins and drain gratings; boiler and furnace parts for domesticheat systems; radiators and refrigerating devices; machine bases andparts; wheels and pulleys and even to small articles which can be castentirely in metal molds. As applied to large y devices it isadvantageous in enabling the use plied to a casting which is partlychilled and partly gray is the production of an ultimate article whichis partly toughened and partly unchanged, free from liability tobreakage, and

susceptible of ready machining in all its parts, since the toughenedportions are essentially steel having graphite inclusions and can bereadily worked. As applied to small-sized articles, which can be made inall-metal molds, my invention is superior to the process known asmalleableizing in that thecost and deterioration of expensive containersis no longer necessary and the strength of the resulting product isenhanced due to the peculiar after-treatment which I have described. Itwill therefore be understood that I do not limit myself in respect ofproducts, Weights, sizes, shapes, temperatures, or modes of treatmentexcepting as specifically recited in my several claims, which I desiremay be construed broadly each independently of limitations contained inother claims.

I claim:

1. The process of producing a metal article of integral section whichconsists in casting an unstable mixture of from 2.50% to 3.50% carbon,1.30% to .60% silicon, and iron under chilling and non-chillingconditions, heating the article to a temperature between 1600 and 1850Fahrenheit to produce in the chilled part of the article a brittlecomposition containing nodular graphite, quickly quenching the articlethrough the critical temperature to retain the graphite structure in thebrittle composition, heating the article to a temperature below thecritical temperature to reduce the brittleness ofthe chilled part of thearticle and render this part tough, and in then cooling the article toatmospheric temperature, the heating, quenching and cooling of saidarticle not materially affecting the unchilled part thereof.

2. The process of producing a metal article of integral section whichconsists in casting an unstable mixture of from 2.50% to 3.50% carbon,1.30% to .60% silicon, and iron under chilling and non-chillingconditions, heating the article to a temperature between 1600 and 18509Fahrenheit for a period of from fifteen minutes to eight hours, thenquickly quenching the article through the critical temperature, andsubsequently reheating the article to a temperature below the criticaltemperature for aperiod of from two to four hours and then cooling thearticle to atmospheric temperature.`

3. The process of producing a. metal article of 125 integral sectionwhich consists in casting an unstable mixture of from 2.50% to 3.50%carbon, 1.30% to .60% silicon, and iron under chillingl and non-chillingconditions, subjecting the article to a temperature between 1600 and1850 130 Fahrenheit, cooling it to a temperature above the criticaltemperature, maintaining the article at this temperature for such lengthof time as to enable the carbon content of the chilled part to becomenormal for thatl temperature, quickly quenching the article through thecritical temperature to a temperature immediately below said criticaltemperature, maintaining the article at a temperature between 600Fahrenheit -and the critical temperature to segregate graphite innodular form embracedA in a continuous phase of ferrite in the chilledpart, and in then cooling the article to atmospheric temperature, theheating, quenching, and cooling of such article not materially affectingthe unchilled part thereof.

4. The process of producing a metal article of integral section whichconsists in casting an unstable mixture of from 2.50% to 3.50% carbon,1.30% to .60% silicon, and iron under chilling and non-chillingconditions, subjecting the article to a temperature between 1600' and1850 Fahrenheit, cooling the article to a temperature immediately abovethe critical temperature, maintaining the article at this temperaturefor about one hour, quenching the article through the criticaltemperature for a period of about four minutes to a. temperatureimmediately below the critical temperature. maintaining the article at atemperature between 600 Fahrenheitv and the critical temperature for aperiod of from two to four hours, and in then cooling the article toatmospheric temperature, the heating. quenching, and cooling of thearticle not materially aiecting the unchilled part thereof.

- FRANK A. FAHRENWALD.

